Hydraulic motors are commonly used as operators for valves in pipeline systems transporting fluids such as oil or gas. For rotary valves the hydraulic motor has a vaned rotor keyed to the valve stem and rotating through an arc of about 90.degree. to rotate the valve element between open and closed positions. Stops are provided in the valve to stop the element accurately at fully open and fully closed positions, and stops in the motor to limit the travel of the vanes must be located accurately to correspond to the stops in the valve.
It is often desirable, particularly in power oil systems, to use solenoid or other electrically operated valves to control the flow of fluid to and from the motor, and limit switches for actuating the solenoid valves are installed in the hydraulic motor to contact the rotor vanes at the end of the valve opening and closing stroke. These switches, particularly the magnetic type usually used, are inaccurate in timing, with the result that it is very difficult to position the switches so as to actuate the solenoid valves at the exact instant required to stop the vanes and the valve element at the fully closed or fully open position. If there is undertravel of the vanes the valve element may not be fully closed or fully open, and if there is overtravel of the vanes, the stops in the valve are subjected to excessive impact and wear.